1. Field of the Invention
This invention relates to an optical coupler device comprising a coupler formed on an optical waveguide.
2. Description of the Prior Art
The optical communication technique whereby light is transmitted while being confined in a medium has achieved a rapid development by the advent of low loss optical fiber. Along therewith, a technique based on the concept of the integrated optics whereby a planar dielectric thin film is used as an optical waveguide and the interior of the thin film is endowed with the function of a lens, a filter or the like to thereby realize an optical integrated circuit in a sufficiently small form as compared with the conventional optical system is absorbing much interest. Such optical integrated circuit of course permits size reduction of the system as well as minimizing the problems peculiar to the conventional optical technique such as disturbance like vibration and reproducibility, and also has a great advantage in reducing the cost.
Applicant has already proposed in U.S. Pat. No. 4,425,023 an integrated optical structure 1 as shown in FIG. 1 of the accompanying drawings. In this integrated optical structure 1, an optical coupler 4 comprising a prism coupler, an interdigital electrode 5 and a thin film lens 6 are provided on a thin film waveguide 3 formed in a planar shape on a substrate 2 placed on XZ plane. An incident light beam L.sub.1 is directed as a light beam L.sub.2 into the waveguide 3 through the optical coupler 4. The light beam L.sub.2 conducted through the waveguide 3 is diffracted and deflected by an ultrasonic wave surface elastic wave W energized by the interdigital electrode 5 provided on a part of the waveguide 3 and becomes a light beam L.sub.3. This deflected light beam L.sub.3 is condensed by the thin film lens 6 so that it forms a calescence point (beam spot) S on the exit end surface 7 of the waveguide 3. That is, the end surface 7 is formed at a position substantially coincident with the focal plane of the thin film lens 6 having a power in the XZ plane parallel to the waveguide 3, and the condensed light beam L.sub.4 is condensed at or near the end surface 7 in X direction substantially orthogonal to the direction of travel of the light beam and exits therefrom. The distribution of light in the Y direction perpendicular to the XZ plane is limited by the thickness d of the waveguide 3 which is usually several .mu.m.
In the integrated optical structure 1 of such configuration, the frequency of a high-frequency voltage applied to the interdigital electrode 5 is varied to change the wavelength of the ultrasonic wave surface elastic wave W propagated on the waveguide 3, whereby the deflection angle of the light beam L.sub.3 is controlled and calescence point scanning is effected on the exit end surface 7. Thus, the integrated optical structure 1 is constructed compactly because a light deflector and a condenser lens are provided on the same substrate 2 and a calescence point S is formed and scanned on or near the exit end surface 7 of the waveguide 3.
The components of this integrated optical structure 1 will be described in greater detail. The substrate 2 may suitably be formed of a material which has a piezoelectric effect and through which the ultrasonic wave of high frequency can be efficiently propagated, and the substrate 2 should desirably be formed of LiNbO.sub.3 (lithium niobate), LiTaO.sub.3 (lithium tantalate) or ZnO (zinc oxide). As regards the waveguide 3, where the substrate 2 is formed of lithium niobate, Ti is diffused under a high temperature of 1000.degree. C. and formed to a thickness of several .mu.m on the substrate 2. Where the substratc 2 is formed of lithium tantalate, the waveguide 3 is obtained by diffusing Nb or Ti. Further combinations may be mentioned, but it is preferable that the waveguide 3 be formed of a material having a high refractive index and a great difference in refractive index from the substrate 2 and capable of conducting light even if the waveguide 3 is made thin. The high refractive index of the waveguide 3 enables the calescence point S formed on the end surface 7 by the thin film lens 6 to be very small in spot diameter, that is, very sharp.
However, the thickness of the dielectric thin film forming the thin film waveguide 3 is of the same degree as the wavelength of the light to be propagated and this leads to a difficulty in efficiently coupling the light into the waveguide 3. Therefore, various methods of coupling the waveguide 3 and the light wave have heretofore been proposed. These methods chiefly include the method of butt-edge coupling from the waveguide end surface and the method using an internal reflection prism or an optical diffraction grating. The butt-edge coupling method is not often adopted because of the problems such as the smoothness of the coupling end surface and the stability of the mechanical arrangement, and actually the method using a prism or a diffraction grating as shown in FIG. 1 has been utilized.
In a conventional optical coupler device using the prism coupling method, it is a requisite condition to provide a medium (e.g. air gap layer 12) lower in refractive index than a prism 10 and the waveguide 3 between the bottom surface 11 of the prism 10 and the surface of the waveguide 3 and couple the light wave by the total reflection of the bottom surface 11 of the prism 10 through said medium, as shown in FIG. 2 of the accompanying drawings. Actually, as shown in FIG. 3 of the accompanying drawings, the prism 10 is pressed by a holder 13, whereby a minute air gap layer 12 less than 1/2 of the wavelength between the bottom surface 11 and the waveguide 3 is utilized to couple the light wave into the waveguide 3. In an optical coupler device using the coupling method utilizing a diffraction grating, a diffraction grating is formed on the waveguide 3 by a hologram sensitizer or photoresist or a diffraction grating is formed directly on the waveguide 3 by an ion beam or the like, whereby coupling is effected by the diffracted light of the light incident on the diffraction grating.
However, in the former optical coupler device using the prism coupling method, it is very difficult to maintain the air gap always stable because the air gap must be kept at about 1/2 or less of the wavelength used and further, there is much possibility of the waveguide 3 being damaged by the bottom surface 11 because the prism 10 is pressed from above it. Also, the use of the holder 13 makes the device bulky and this in turn leads to the disadvantage that the characteristic of compactness of the optical integrated circuit cannot sufficiently be displayed.
The coupling method using a diffraction grating is superior in compactness to the prism coupling method, but when it is considered that various function elements must be mounted on the waveguide 3 as in the aforedescribed integrated optical structure 1, it is difficult to form a diffraction grating on the waveguide 3. Also, to enhance the coupling efficiency, it is necessary that light be made to be incident from the substrate 2 side or the shape of the diffraction grating be made into an asymmetric blazed grating or the like. In the former case, operability is poor and in the latter case, such grating is difficult to form and the yield of manufacture of the optical coupler device is low due to the failure in forming the grating and the difficulty of resuscitation from the failure.